This invention pertains to the art of refrigeration and, in particular, to an arrangement promoting refrigerating efficiency through the provision of a structure which combines in a particular way a refrigerant liquid receiver, a refrigerant suction gas accumulator, and a liquid-suction heat exchanger.
In a transport refrigeration system, such as that provided by the assignee of this invention, and as somewhat schematically shown in the prior art FIG. 1, liquid refrigerant passes through the outlet of the condensing coil 10 to a receiver tank 12. The liquid refrigerant then passes through line 14 to a liquid refrigerant, suction gas heat exchanger 16 en route to expansion valve 18 and into the inlet of the evaporator coil 20. The suction gas leaving the evaporator coil is routed through line 22 to heat exchanger 16 and from there to line 24 to an accumulator tank 26 which, in its conventional form, includes the U-shaped dip tube 28 through which the vaporous refrigerant is drawn into line 30 which connects to the suction inlet of the compressor 32.
As may be seen in FIG. 1, the evaporator 20 and heat exchanger 16 are located within the confines of the conditioned space such as the trailer 34, while both the condenser 10 and the accumulator 26 are located in a cabinet 36 exterior of the trailer, and subject to ambient temperatures. Typically, the temperature within the cabinet 36, and to which the receiver 12 is subjected, will be even higher than the ambient temperature outside of the cabinet since the heat from the condenser 10 and from the radiator for the engine driving the compressor 32 add to the heat from the outdoors. Because of the relatively high ambient temperature in the vicinity of the receiver 12 and the related piping 14, the subcooled refrigerant liquid leaving the condenser coil is reheated. Thus the cooling capacity of the system is diminished to the extent that the liquid refrigerant is heated by the warm ambient surroundings. The heat exchanger 16 is intended to reduce this problem by transferring heat from the warm liquid refrigerant to the cooler vaporous refrigerant.
In passing from the heat exchanger 16 to the compressor 32, the suction gas is routed through the accumulator tank 26, as previously noted. The accumulator serves its normal function as a reservoir for liquid refrigerant and to prevent the passage of any significant amount of liquid refrigerant to the compressor. In the transport refrigerant environment, the accumulator also functions in the fashion of an evaporator when the reversible unit is operating in a heating mode, as distinct from a cooling mode. To have the accumulator function as an evaporator during the heating mode, means is provided to deliver heat to the lower portion of the accumulator, and this may be accomplished such as by providing tubes 38 coiled around the lower portion of the accumulator and connected to the engine coolant circuit.
The cooling capacity and efficiency of the system in the prior art of FIG. 1 also suffers to a degree from heating of the accumulator 26 by warm ambient temperatures in the cabinet 36. The warm ambient causes the refrigerant vapor to be superheated to a temperature well above the temperature of saturated vapor. To the degree that this happens, the system is penalized.
As a typical example of how the system is penalized with relatively high outdoor air temperatures, typical examples of temperature values will be given. If the outside air temperature is about 100.degree. F. (38.degree. C.), the air temperature around the receiver may be significantly hotter, such as 135.degree. F. (57.degree. C.) because of heat given off by the engine radiator and the condenser. The hot refrigerant liquid received by the receiver 12 may be in the order of 115.degree. F. (46.degree. C.) so the refrigerant in the receiver and in its passage through line 14 to heat exchanger 16 is heated, which is a penalty to the system.
Under the temperature conditions assumed, the vaporous refrigerant leaving the evaporator 20 and passing to the heat exchanger may be, say, 10.degree. F. (-12.degree. C.) where it is perhaps heated to, say, 65.degree. F. (18.degree. C.), at which temperature it passes to the accumulator 26. With the relatively high ambient of, say, 135.degree. F. (57.degree. C.), the refrigerant vapor may be heated up to, say, 90.degree. F. (32.degree. C.) in the accumulator and in its passage to the compressor. Thus the vapor is highly superheated under these conditions, well beyond the degree of superheat leaving the liquid-suction heat exchanger, and this high superheat also penalizes the system.
The compressor cooling efficiency in this prior art system is also penalized by the suction line restriction that occurs in the U-tube 28 within the accumulator tank 26. This suction restriction is due to the combined effect of the entrance loss at the U-tube inlet and the partial internal obstruction by the liquid lubricating oil which tends to collect in the bottom of the U-tube.
A further problem with the prior art system relates to the return of lubricating oil to the compressor crankcase. The oil aerosol that returns to the compressor 32 entrained with the suction vapor is expected to separate within the compressor inlet passages and drain back to the compressor crankcase. Because of relatively high vapor transport velocities within the compressor inlet passages, an undesirable proportion of this returned oil remains entrained in the vapor and is recycled through the entire system. This penalizes the total performance by reduced compressor pumping efficiency and by reduced heat transfer within the condenser 10 and evaporator 20 coils.
It is the aim of this invention to mitigate the problems noted through the provision of a structural arrangement of the receiver, accumulator, and heat exchanger in combination.